The need for an efficient, non-polluting power source for vehicles and stationary power plants in urban environments has resulted in increased attention to the option of fuel-cell systems of high efficiency and low emissions. Hydrogen is the most suitable fuel for a fuel cell system, providing the highest conversion efficiency for fuel-on-board-to-electric-power for vehicular systems and generating zero emissions since water is the only product of the hydrogen/air fuel cell process. In the hydrogen/air fuel cell, the processes at the anode and cathode, respectively, are:H2=2H++2e (anode process)  (1)and,O2+4e+4H+=2H2O (cathode process)  (2)
Hydrogen fuel could be carried on board the vehicle or stored as either neat hydrogen, in the form of pressurized gas or cryogenically stored liquid, or in the form of a more ordinary liquid fuel, such as methanol or liquid hydrocarbon, which needs to be processed/converted on board the vehicle to a mixture of hydrogen and CO2. Because hydrogen is difficult or expensive to store, it likely that fuel processors will be employed to convert hydrocarbons or oxygenates to hydrogen for vehicle and for stationary power generation systems in an integrated fuel processor/fuel cell system.
Hydrogen may be produced from hydrocarbons or oxygenates in a fuel processor zone which generally consists of a steam reforming zone, a steam reforming zone and a partial oxidation zone (secondary reforming) or autothermal reforming zone (partial oxidation and steam reforming) to convert the hydrocarbon or oxygenate feed stream into a synthesis gas stream. Modifications of the simple steam reforming processes have been proposed to improve the operation of the steam reforming process. In particular, there have been suggestions for improving the energy efficiency of such processes in which the heat available from the products of a secondary reforming step is utilized for other purposes within the synthesis gas production process. For example, processes are described in U.S. Pat. No. 4,479,925 B1 in which heat from the products of a secondary reformer is used to provide heat to a primary reformer.
The reforming reaction is expressed by the following formula:CH4+2H2O→4H2+CO2 where the reaction in the reformer and the reaction in the shift converter are respectively expressed by the following simplified formulae (3) and (4):CH4+H2O→CO+3H2  (3)CO+H2O→H2+CO2  (4)
Because formula (3) will produce CO, and CO can be detrimental to the operation of the fuel cell, a series of CO removal steps may be included in a fuel processor zone. One of the most common CO removal or hydrogen purification steps is a water gas shift conversion zone. In the water gas shift conversion zone which typically follows a reforming step, formula (4) is representative of the major reaction.
If it is required to reduce the CO concentration to very low levels, such as less than 50 ppm mol, or less than 10 ppm mol, a preferential oxidation step may follow the water gas shift step. In the preferential oxidation step, the hydrogen fuel stream at effective conditions is contacted with a selective oxidation catalyst in the presence of an oxygen containing stream to selectively oxidize carbon monoxide to carbon dioxide and produce a fuel stream comprising between about 10 and 50 ppm-mol carbon monoxide. The thus purified fuel stream is passed to an anode side of the fuel cell and an air stream is passed to the cathode side of the fuel cell.
Others have attempted to combine the operation of a fuel cell and a fuel processor by thermally integrating these components and supplying the heat requirements of the fuel processor by integrating the endothermic steam reforming reaction zone with exothermic zones including reaction zones for partial oxidation and combustion zones. Generally, the focus of the prior developments was on the distribution of fuel to these zones in order to balance the essentially static operation of the combined system. By the term “essentially static operation”, it is meant that the output of electric power from the fuel cell varies by less than about 5 to 10 percent. Ancillary equipment, such as an air compressor or blower is required to supply air to the air consuming zones of the process. The air consuming zones of the combined fuel cell/fuel processor system generally include a combustion zone for burning an anode waste gas stream from the fuel cell to provide heat to an endothermic reaction zone and a partial oxidation zone for converting a portion of the feed stream to produce the fuel stream for the fuel cell. A conventional approach to supplying the air to these zones would result in a very complex and expensive arrangement of compressors, valves, and sensors for each air consuming zone. Air distribution methods are sought to supply air to air consuming zones of integrated fuel processor/fuel cell systems which is consistent with low complexity and high reliability.